The prospect of Trump or Cruz at the top of the ticket makes Republicans holding onto control of the Senate much more difficult. (Chip Somodevilla/Getty Images)

With Donald Trump and Texas Sen. Ted Cruz seemingly positioned to fight it out for the Republican presidential nomination, Democrats are now poised to take over the Senate in November.

The two Republicans still in the race who could help their party’s Senate prospects, Florida Sen. Marco Rubio and Ohio Gov. John Kasich, continue to flounder. While a deadlocked GOP convention in Cleveland could, at least in theory, nominate a candidate with broad appeal and low enough negatives to revive the party’s Senate prospects, that development is both a long way in the future and a long shot.

Obama listens to a question from Pintal County, Ariz., Sheriff Paul Babeu during a town hall event on Thursday. (Aude Guerrucci/Pool/Getty Images)

In the heat of his push for more gun control, President Barack Obama threatened to withhold support from anyone, including Democrats, who didn’t support “common-sense” changes. But based on the political realities of this cycle, his comments aren’t likely to dramatically impact Senate races.

“Even as I continue to take every action possible as president, I will also take every action I can as a citizen,” Obama wrote in a New York Times op-ed . “I will not campaign for, vote for or support any candidate, even in my own party, who does not support common-sense gun reform.”

Costello is potentially vulnerable in his first re-election race, which will take place in a presidential year in a competitive district. (Bill Clark/CQ Roll Call File Photo)

Democrats have had their eyes on Pennsylvania’s 6th District for over a decade, but haven’t been able to crack the code to win the seat.

They thought that had a top-tier recruit in Mike Parrish, a businessman, military veteran, and former Republican. But nearly halfway through the cycle, Parrish’s campaign has floundered and Democratic strategists are moving on and searching for another candidate.

Johnson's re-election race, along with those of Ayotte and Toomey, are at the core of the battle for control of the Senate. (Tom Williams/CQ Roll Call File Photo)

There is little doubt about the identity of the most vulnerable senator seeking re-election next year. It’s Illinois Republican Mark S. Kirk, who hopes to win a second term in a very Democratic state in a presidential year. His prospects are bleak.

But who ranks just behind Kirk as the second most vulnerable senator up next year?

Feingold made trade a pillar of his attacks on Johnson in 2010. (Tom Williams/CQ Roll Call File Photo)

The White House on Thursday released the much anticipated text of the Trans-Pacific Partnership agreement — a trade deal that will likely play out in a number of top House and Senate contests in 2016, whether or not it's passed by Congress.

Most of those races are located in the Rust Belt — states where past trade pacts such as the North American Free Trade Agreement from the 1990s are often blamed for the sharp decline in manufacturing jobs that once made the region prosper.

Jolly might've faced a competitive race had Democrats not cleared the field for a candidate who dropped out. (Bill Clark/CQ Roll Call File Photo)

Former Republican state Sen. Tom O’Halleran announced Tuesday he was running as a Democrat in Arizona’s 1st District. It’s not the first time party strategists have crossed the aisle to recruit, and O’Halleran isn’t even the only party-switcher running this cycle.